1. Field of the Invention
The present invention relates to a light unit, and more particularly to a backlight unit. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving brightness uniformity and simplifying the fabrication process of a backlight unit.
2. Description of the Related Art
In generally, there is a trend in which the range of applications for a liquid crystal display device is broadening due to its characteristics of light weight, thin profile, and low driving power consumption. In keeping with this trend, the liquid crystal display device is now being used in office automation equipment and home audio/video equipment. The liquid crystal display device controls the transmitted amount of a light beam in accordance with a video signal applied to a plurality of control switches which are arranged in a matrix shape, thereby displaying a desired picture on a screen. Thus, the liquid crystal display device requires a separate light source, such as a backlight, because the liquid crystal display device in and of itself is not a self-luminous display device.
There are two types of backlight for a liquid crystal display device depending on where the light source is located on the liquid crystal display device. The first type is a direct type and the second type is the edge type. The edge type backlight has a light source installed at an edge of one side of a liquid crystal display device, and irradiates light from the light source to a liquid crystal display panel through a light guide panel and a plurality of optical sheets. The direct type backlight has a plurality of light sources disposed directly under the liquid crystal display device, and irradiates the light from the light sources to the liquid crystal display panel through a diffusion plate and a plurality of optical sheets. A cold cathode fluorescent lamp (CCFL) and a light emitting diode (LED) are typically used as a light source in a backlight. The direct type backlight, which has higher brightness, light uniformity and color purity than the edge type backlight, is mostly used in an LCD TV.
FIG. 1 is a cross-sectional diagram representing a liquid crystal display module of the related art. As shown in FIG. 1, a related art liquid crystal display device includes a liquid crystal display panel 11 for displaying a picture; and a backlight unit 10 for irradiating light onto the liquid crystal display panel 11. A plurality of data lines and a plurality of scan lines are arranged to cross each other in the liquid crystal display panel 11 to define liquid crystal cells arranged in an active matrix shape between upper and lower substrates. Further, pixel electrodes are formed on the lower substrate and a common electrode is formed on the upper substrate such that an electric field can be applied across each of the liquid crystal cells. Thin film transistors (TFTs) are formed adjacent to where the data lines and gate lines cross. The TFTs apply a data signal to a pixel electrode in response to a scan signal on a gate line connected to the TFT. Gate drive IC's and data drive IC's are electrically connected through a tape carrier package (TCP) to the gate and data lines of the lower substrate.
The backlight unit 10 includes a bottom cover 13, a lower reflection plate 14, an intermediate light guide panel 15, a diffusion plate 16, a plurality of light emitting diodes 17 and a plurality of optical sheets 12. The bottom cover 13 includes a bottom surface and side surfaces. The lower reflection plate 14 is within the side surfaces of the bottom cover 13. The lower reflection plate 14 has holes in which the light emitting diodes 17 pass through.
The light emitting diodes 17 include a red light emitting diode, a green light emitting diode and a blue light emitting diode, and emit light in response to a current supplied from a light source drive circuit (not shown) so as to generate red, green and blue lights. The intermediate light guide panel 15 is formed of a transparent plastic material and disposed between the light emitting diodes 17 and the diffusion plate 16. Intermediate reflection plates 18 corresponding to the light emitting diodes are on a bottom surface of the intermediate light guide panel 15, which corresponds to a location on the intermediate light guide panel 15. The intermediate reflection plates 18 are diverters that reflects the lights of red, green and blue which are directly irradiated from the light emitting diodes 17.
The red, green and blue lights, which radiate from the light emitting diodes 17 by way of the lower reflection plate 14, the intermediate reflection plate 18 and the light guide panel 15 are mixed, and as a result, a white light is incident onto the optical sheets 12 through the intermediate light guide panel 15 and the diffusion plate 16. The intermediate light guide panel 15 is assembled to the bottom cover 13 along with the diffusion plate 16. The diffusion plate 16 includes a plurality of beads that disperse the light from the intermediate light guide panel 15. The beads disperse the light such that there is no difference in light distribution across the diffusion plate 16 between locations over the light emitting diodes and elsewhere in the display surface of the liquid crystal display panel 11. The optical sheets 12 include a diffusion sheet and a prism sheet for uniformly irradiating the light from the diffusion plate 16 across the whole surface of the liquid crystal display panel and to reorient the path of the light from the diffusion plate 16 into a perpendicular direction with respect to the display surface so as to increase brightness directly at the front of the display surface.
Such a backlight unit 10 has problems. In larger sizes of this type of liquid crystal display device, the intermediate light guide panel 15 can sag because supporting points for the intermediate light guide panel 15 only exist at the edges. Thus, a gap between the intermediate reflection plates 18 and the light emitting diodes 17 can vary in that the gap is smaller in the center of the liquid crystal display device. As a result, the amount of light incident onto the liquid crystal display panel 11 is reduced and brightness varies because the red, green and blue lights generated by the light emitting diodes 17 are not uniformly mixed. Accordingly, the purity degree of the white light, which is irradiated onto the liquid crystal display panel 11, decreases such that color reproduction characteristic of the liquid crystal display panel 11 can be degraded. Further, a process of adhering the reflector 18 to the intermediate light guide panel 15 is hard to be automated. More specifically, the reflector 18 is attached manually for each LED light source, thereby increasing production cost.